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MeshUniforms on the GPU via compute shader where available. (
#12773) Currently, `MeshUniform`s are rather large: 160 bytes. They're also somewhat expensive to compute, because they involve taking the inverse of a 3x4 matrix. Finally, if a mesh is present in multiple views, that mesh will have a separate `MeshUniform` for each and every view, which is wasteful. This commit fixes these issues by introducing the concept of a *mesh input uniform* and adding a *mesh uniform building* compute shader pass. The `MeshInputUniform` is simply the minimum amount of data needed for the GPU to compute the full `MeshUniform`. Most of this data is just the transform and is therefore only 64 bytes. `MeshInputUniform`s are computed during the *extraction* phase, much like skins are today, in order to avoid needlessly copying transforms around on CPU. (In fact, the render app has been changed to only store the translation of each mesh; it no longer cares about any other part of the transform, which is stored only on the GPU and the main world.) Before rendering, the `build_mesh_uniforms` pass runs to expand the `MeshInputUniform`s to the full `MeshUniform`. The mesh uniform building pass does the following, all on GPU: 1. Copy the appropriate fields of the `MeshInputUniform` to the `MeshUniform` slot. If a single mesh is present in multiple views, this effectively duplicates it into each view. 2. Compute the inverse transpose of the model transform, used for transforming normals. 3. If applicable, copy the mesh's transform from the previous frame for TAA. To support this, we double-buffer the `MeshInputUniform`s over two frames and swap the buffers each frame. The `MeshInputUniform`s for the current frame contain the index of that mesh's `MeshInputUniform` for the previous frame. This commit produces wins in virtually every CPU part of the pipeline: `extract_meshes`, `queue_material_meshes`, `batch_and_prepare_render_phase`, and especially `write_batched_instance_buffer` are all faster. Shrinking the amount of CPU data that has to be shuffled around speeds up the entire rendering process. | Benchmark | This branch | `main` | Speedup | |------------------------|-------------|---------|---------| | `many_cubes -nfc` | 17.259 | 24.529 | 42.12% | | `many_cubes -nfc -vpi` | 302.116 | 312.123 | 3.31% | | `many_foxes` | 3.227 | 3.515 | 8.92% | Because mesh uniform building requires compute shader, and WebGL 2 has no compute shader, the existing CPU mesh uniform building code has been left as-is. Many types now have both CPU mesh uniform building and GPU mesh uniform building modes. Developers can opt into the old CPU mesh uniform building by setting the `use_gpu_uniform_builder` option on `PbrPlugin` to `false`. Below are graphs of the CPU portions of `many-cubes --no-frustum-culling`. Yellow is this branch, red is `main`. `extract_meshes`:  It's notable that we get a small win even though we're now writing to a GPU buffer. `queue_material_meshes`:  There's a bit of a regression here; not sure what's causing it. In any case it's very outweighed by the other gains. `batch_and_prepare_render_phase`:  There's a huge win here, enough to make batching basically drop off the profile. `write_batched_instance_buffer`:  There's a massive improvement here, as expected. Note that a lot of it simply comes from the fact that `MeshInputUniform` is `Pod`. (This isn't a maintainability problem in my view because `MeshInputUniform` is so simple: just 16 tightly-packed words.) ## Changelog ### Added * Per-mesh instance data is now generated on GPU with a compute shader instead of CPU, resulting in rendering performance improvements on platforms where compute shaders are supported. ## Migration guide * Custom render phases now need multiple systems beyond just `batch_and_prepare_render_phase`. Code that was previously creating custom render phases should now add a `BinnedRenderPhasePlugin` or `SortedRenderPhasePlugin` as appropriate instead of directly adding `batch_and_prepare_render_phase`.
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